Motor vehicles use temperature controlled precision resistors, in particular, hot wires or hot films, in air mass meters to measure the mass flow rate of air drawn into an internal combustion engine. The air stream entering the engine flows around the heated wire. The precision resistor, which is part of an electrical bridge circuit, is maintained at a constant operating temperature by timed current pluses. Because the heating current is a function of the temperature of the resistor and, in turn, the mass flow rate of air, the heating current is used as a measure of the mass flow rate of air drawn into the engine. Data corresponding to the mass flow rate of air is supplied to a control unit for setting the optimum operating points for the internal combustion engine.
In a typical method for controlling the temperature of a precision resistor for measuring the mass flow rate of air, an available voltage supply, usually the motor vehicle's power supply or battery voltage, is supplied to a voltage stabilizing circuit. The stabilized voltage is then used to control the timing of the current pulses for heating the precision resistor. Since the stabilized supply voltage is lower than the operating voltage, it takes a relatively long time after the internal combustion engine is started for the precision resistor to reach its operating temperature. Problems arise when the supply voltage drops, for example, due to a weak battery, because the voltage stabilizing circuit operates properly only when a sufficient voltage potential difference exists across its input and output terminals.
It is an object of the present invention, therefore, to overcome the problems of known methods and apparatus for controlling the temperature of a resistor for measuring mass flow rates of air.